Earliest attested in the Southern dynasties poet Xu Ling's 杂曲: 绿黛红颜两相发，千娇百态情无歇 (kohl-blue brows and red cheeks set each other off; a thousand charms, a thousand poses, ceaseless feeling).
usage
Almost always describes women's beauty or, by extension, flowers. Modifying a man would sound effeminate or mocking. 千 (thousand) and 百 (hundred) here just mean 'countless' - the qiān...bǎi... pattern (千变百化, 千恩百谢) is a common idiom pattern.
